1. Field of the Invention
The present invention relates generally to nuclear reactor fuel assemblies and, more particularly, is concerned with the connection between the fuel assembly grids and the control rod guide thimbles.
2. Description of Related Art
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies, each of which is composed of top and bottom nozzles, also referred to as end fittings and a plurality of transverse support grids axially spaced along and attached to the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles, and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission, and thus the release of a large amount of energy in form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generation in the core for the production of useful work. Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during operation, and at shut down is achieved by varying the neutron flux. Generally this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of the neutron absorber control rods within the reactor core. The level of the neutron flux, and thus the heat output of the core is normally regulated by movement of the control rods into and from the guide thimbles.
The guide thimbles are rigidly connected at each end respectively to the top nozzle and the bottom nozzle and the grids are fixably attached to the guide thimbles at grid cell locations through which the guide thimbles pass. The top nozzle, bottom nozzle, guide thimbles and grids thus form the structural elements of the fuel assembly also known as the fuel assembly skeleton.
The grids are used to precisely maintain the spacing between fuel rods in a nuclear reactor core, prevent rod vibration, and provide lateral support for the fuel rods. Grids are made of materials with low neutron absorption cross-sections such as stainless steel, Inconel, and alloys of Zirconium, such as Zircaloy, to minimize grid deformation, neutron capture, and the loss of structural integrity during irradiation. Conventional designs of grids for nuclear fuel assemblies include a multiplicity of interleaved interior grid straps formed in an egg-crate configuration defining cells which accept the fuel rods and the guide thimbles. The ends of each of the interior grid straps are interlocked with an outer grid strap, forming the peripheral cells of the grid. Each cell through which the fuel rods pass provides support to one fuel rod at a given axial location through the use of relatively resilient springs of various forms. In order to minimize the lateral displacement of fuel rods and to improve the fuel characteristics of an assembly, a number of grids are used along the fuel assembly length. In a pressurized water reactor, typically each grid is held in place along the fuel assembly by its attachment to the control rod guide thimbles.
The interior straps of the grids that are interlocked in an egg-crate pattern are generally held in place by a welded or braised joint at their intersecting locations. The ends of the lattice straps are similarly affixed to the perimeter straps that surround them by welds or braises. If the straps are made of Zircaloy or stainless steel, they can generally be welded. If Inconel or nickel plated Inconel are employed, they generally have to be braised. Various means of attachment are used to position and secure the spacer grid assemblies to the guide thimble tubes. These means of attachment include welding of the grids to the tubes, braising, bulging of the tubes into sleeves that are attached to the grids, and welding split rings 40 to the guide thimbles 18 directly above and below the grid straps 42 as shown in FIG. 1. The latter two mechanical approaches to connecting the guide thimbles to the grid straps are necessary where dissimilar materials are employed for the grid straps and the guide thimbles, e.g., Inconel grids and Zircaloy guide thimbles. With individual split rings 40 used on either side of the grid straps 42, issues can arise due to the size of the gaps between the rings 40 and the grid straps 42, which can result in uneven loading of the rings 40, lack of coplanarity of the rings 40, and difficulties inspecting the ring to grid gaps.
Designers are constantly seeking to improve the means of manufacture of the grids and fuel assembly skeletons. Areas of interest include mechanisms for reducing the manufacturing effort, and meeting the stringent design envelope, or tolerances, on dimensional parameters of the grid. Further considerations include retaining the structural rigidity of the fuel assembly skeleton. More particularly, the need specifically exists for an improved connection between the grid straps and the guide thimbles that will accommodate the use of dissimilar materials for the grid straps and the guide thimbles. While Zircaloy has a lower neutron capture cross section than Inconel, Inconel has a greater stiffness and a lower relaxation rate than Zircaloy and thus is more desirable for use as a grid strap material. U.S. patent application Ser. No. 11/764,540, filed Jun. 18, 2007 and assigned to the assignee of this invention addresses that need by providing a through grid split sleeve that extends from above the grid straps to a distance below the grid straps. The uncompressed diameter of the sleeve is equal to or larger than the diameter of the cell through which the guide thimble extends. Preferably, the sleeve is made of a resilient material that is the same as or weld compatible to that of the guide thimble. The sleeve is compressed and in its compressed condition it is inserted through the corresponding cell of the grid with a portion of the sleeve extending above and below the grid straps. The sleeve is then allowed to expand to its uncompressed condition thereby securing the sleeve within the guide thimble cell. The sleeve may then be welded or otherwise secured to the grid strap. When the sleeve is secured within the cell, the guide thimble can be inserted and welded or otherwise attached at either end of the sleeve or at both ends of the sleeve. While this arrangement provides a secure connection between the grid straps and the guide thimbles when employing dissimilar materials further means are still desired that will enhance the rigidity of the connection.